Method of depositing transition metal nitride thin films

ABSTRACT

This invention concerns a method for depositing transition metal nitride thin films by an Atomic Layer Deposition (ALD) type process. According to the method vapor-phase pulse of a source material, a reducing agent capable of reducing metal source material, and a nitrogen source material capable of reacting with the reduced metal source material are alternately and sequentially fed into a reaction space and contacted with the substrate. According to the invention as the reducing agent is used a boron compound which is capable of forming gaseous reaction byproducts when reacting with the metal source material.

REFERENCE TO RELATED APPLICATIONS

The present application is the U.S. national phase of internationalapplication number PCT/FI00/00895 and claims priority under 35 U.S.C.§119 to Finnish application number 19992234, filed Oct. 15, 1999.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to metal nitride thin films. Inparticular, the invention concerns a method of growing tungsten nitridethin films by Atomic Layer Deposition (referred to as ALD hereinafter).

2. Description of Related Art

The integration level of components in integrated circuits isincreasing, which rapidly brings about a need for a decrease of the sizeof components and interconnects. Design 15 rules are setting the featuresizes to ≦0.2 μm. Complete film coverage on deep bottoms and vias isbard to obtain.

Integrated circuits contain interconnects which are usually made ofaluminium or copper. Especially copper is prone to diffusion to thesurrounding materials. Diffusion affects the electrical properties ofthe circuits and active components may malfunction. The diffusion ofmetals from interconnects into active parts of the device is preventedwith an electrically conductive diffusion barrier layer. Favoreddiffusion barriers are, e.g., amorphous transition metal nitrides, suchas TiN, TaN and WN. The nitrides can be non-stoichiometric becausenitrogen is located in interstitial position of the lattice.

In the Chemical Vapor Deposition method (referred to CVD hereinafter),the source materials arc typically fed to reaction space together, andthey react with each other when brought into contact with the substrate.It is also possible to feed one source material containing all desiredreactant species to a CVD reactor, and heat it almost to a point whereit decomposes thermally. When the heated gas contacts the substratesurface, a cracking reaction occurs and a film is grown. As is apparentfrom the above discussion, in CVD the concentration of the differentsource materials in the reaction space determines the growth of thefilm.

Atomic Layer Deposition (ALD) and, originally, Atomic Layer Epitaxy(ALE) is an advanced variation of CVD. The method name was changed fromALE into ALD to avoid possible confusion when discussing aboutpolycrystalline and amorphous thin films. The ALD method is based onsequential self-saturated surface reactions. The method is described indetail in U.S. Pat. Nos. 4,058,430 and 5,711,811. The reactor designbenefits from the usage of inert carrier and purging gases which makesthe system fast.

The separation of source chemicals from each other by inert gasesprevents gas-phase reactions between gaseous reactants and enablesself-saturated surface reactions leading to film growth which requiresneither strict temperature control of the substrates nor precise dosagecontrol of source chemicals. Surplus chemicals and reaction byproductsare always removed from the reaction chamber before the next reactivechemical pulse is introduced into the chamber. Undesired gaseousmolecules are effectively expelled from the reaction chamber by keepingthe gas flow speeds high with the help of an inert purging gas. Thepurging gas pushes the extra molecules towards the vacuum pump used formaintaining a suitable pressure in the reaction chamber. ALD provides anexcellent and automatic self-control for the film growth.

ALD has recently been used for depositing single layers of titaniumnitride TiN (H. Jeon, J. W. Lee, J. H. Koo, Y. S. Kim, Y. D. Kim, D. S.Kim, “A study on the Characteristics of TiN Thin Film Deposited byAtomic Layer Chemical Vapor Deposition method”, AVS 46^(th)International Symposium, abstract TF-MoP17,http://www.vacuum.org/symposium/seattle/technical.html, to be presentedOct. 27, 1999 in Seattle, USA).

According to Hiltunen et al. NbN, TaN, Ta₃N₅, MoN and Mo₂N can be grownby ALD using metal halogenides as source chemicals (L. Hiltunen, M.Leskelä, M. Mäkelä, L. Niinistö, E. Nykänen, P. Soininen, “Nitrides ofTitanium, Niobium, Tantalum and Molybdenum Grown as Thin Films by theAtomic Layer Epitaxy Method”, Thin Solid Films, 166 (1988) 149-154). Theuse of additional zinc vapour during the deposition has decreased theresistivity of the nitride film either by increasing the metal/nitrogenratio or by removing oxygen from the films.

J. W. Klaus has disclosed a process for growing tungsten nitride filmsusing an ALD method (J. W. Klaus, “Atomic Layer Deposition of Tungstenand Tungsten Nitride Using Sequential Surface Reactions”, AVS 46^(th)International Symposium, abstract TF-TuM6,http://www.vacuum.org/symposium/seattle/technical.html, to be presentedOct. 26, 1999 in Seattle, USA). In the process of the publication,tungsten nitride W₂N is grown from WF₆ and NH₃.

In the art, tungsten compounds have been reduced by using hydrogen (H₂)U.S. Pat. No. 5,342,652 and EP-A2-899 779), silanes, such as SiH₄ (U.S.Pat. No. 5,691,235) and chlorosilanes, such as SiHCl₃ (U.S. Pat. No.5,723,384).

There are, however, drawbacks related to these prior art methods.Silanes may also react with WF₆, thus forming tungsten silicides, suchas WSi₂. Hydrogen can reduce a tungsten compound into tungsten metalwhich has too low vapor pressure for being transported in gas phase ontosubstrates. Traditional CVD processes may leave significant amounts ofimpurities in thin films, especially at low deposition temperatures.

SUMMARY OF THE INVENTION

It is an object of the present invention to eliminate the problems ofthe prior art and to provide a novel method of depositing transitionmetal nitride thin films by an ALD type process. It is a further objectof the invention to provide a process for preparing diffusion barrierson metal surfaces in integrated circuits.

The invention is based on the surprising finding that by feeding into areactor chamber, which contains a substrate, a suitable transition metalcompound and, a reducing boron compound pulse and a nitrogen compound, ametal nitride film with low resistivity can be grown. According to thepresent invention, the reaction between the gaseous boron compound andthe metal species reduces the metal compound and gives rise to gaseousreaction byproducts, which easily can be removed from the reactionspace.

According to a preferred embodiment of the invention, the metal nitridethin films are grown by an ALD type process. This is carried out bysequentially feeding into a reactor chamber, which contains a substrate,alternate pulses of a suitable transition metal compound, a reducingboron compound pulse and a nitrogen compound, said boron compound andsaid nitrogen compound being fed after the metal compound. Thus, a metalnitride film with low resistivity can be grown in accordance with theprinciples of ALD method. According to the present invention, thereaction between the gaseous boron compound and the metal species boundto the surface reduces the metal compound and gives rise to gaseousreaction byproducts, which easily can be removed from the reactionspace.

A diffusion barrier can be grown in an integrated circuit by depositing,during the manufacture of the integrated circuit, a metal nitride thinfilm on a dielectric surface or a metal surface present on the siliconwafer blank.

More specifically, the present method is characterized by what is statedin the characterizing part of claim 1.

The process for preparing diffusion barriers is characterized by what isstated in the characterizing part of claim 20.

A number of considerable advantages are achieved with the aid of thepresent invention Metal nitride thin films, in particular tungstennitride thin films, can be grown at low temperatures. The boroncompounds used as source materials are easy to handle and vaporise.

As mentioned above, the boron compounds formed as byproducts of thereaction between the metal species and the reducing boron compound areessentially gaseous and they exit the reactor easily when purging withan inert gas. The boron residues in the film are on a very low level,typically below 5 wt-%, preferably 1 wt-% or less and in particular 0.5wt-% or less. The resistivity of the film is low. The growing rate ofthe film is acceptable. Also the reaction times are short, and in all itcan be said that films can be grown very effectively by means of thepresent process.

The film grown with the present process exhibits good thin filmsproperties. Thus, especially the metal nitride films obtained by an ALDtype process have an excellent conformality even on uneven surfaces andon trenches and vias. The method also provides an excellent andautomatic self-control for the film growth.

The metal nitride thin films grown by the present invention can be used,for example, as ion diffusion barrier layers in integrated circuits.Tungsten nitride stops effectively oxygen and increases the stability ofmetal oxide capacitors. Transition metal nitrides and especiallytungsten nitride is also suitable as an adhesion layer for a metal, as athin film resistor, for stopping the migration of tin through via holesand improving the high-temperature processing of integrated circuits.

Next, the invention is described in detail with the aid of the followingdetailed description and by reference to the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 presents a block diagram of a pulsing sequence according to apreferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

For the purposes of the present invention, a “chemical gaseousdeposition process” designates a deposition process in which thereactants are fed to a reaction space in vapor phase. Examples of suchprocesses include CVD and ALD.

For the purposes of the present invention, an “ALD type process”designates a process in which deposition of vaporized material onto asurface is based on sequential self-saturating surface reactions. Theprinciple of ALD process is disclosed, e.g., in U.S. Pat. No. 4,058,430.

“Reaction space” is used to designate a reactor or reaction chamber inwhich the conditions can be adjusted so that the deposition by ALD ispossible.

“Thin film” is used to designate a film which is grown from elements orcompounds that are transported as separate ions, atoms or molecules viavacuum, gaseous phase or liquid phase from the source to the substrate.The thickness of the film depends on the application and it varies in awide range, e.g., from one molecular layer to 800 nm, even up to 1000nm.

The Deposition Process

According to the present invention, metal nitride thin films areprepared by ALD type process.

According to CVD process, a film is grown on a substrate placed in areaction chamber at elevated temperatures. The principles of CVD arewell known to those skilled in the art. The metal source material, thenitrogen source material and the reducing boron compound are typicallyfed to the reaction space essentially simultaneously, although theduration of the pulsing of the different species may vary. It is alsopossible to feed a source material comprising both the nitrogen andmetal to the reaction space together with the reducing boron compound.

According to the present invention, metal nitride thin films areprepared by the ALD process. Thus, a substrate placed in a reactionchamber is subjected to sequential, alternately repeated surfacereactions of at least two vapor-phase reactants for the purpose ofgrowing a thin film thereon. Metal compounds used as source materialsare reduced by boron compounds on a substrate maintained at an elevatedtemperature. The boron compounds, on the other hand, are notincorporated into the film. The reduced metal species react on thesurface with gaseous or volatile nitrogen source material.

The conditions in the reaction space are adjusted so that no gas-phasereactions, i.e., reactions between gaseous reactants, occur, onlysurface reactions, i.e., reactions between species adsorbed on thesurface of the substrate and a gaseous reactant. Thus, the molecules ofthe reducing boron compound react with the deposited metal sourcecompound layer on the surface, and the nitrogen source material reactswith the reduced metal compound on the surface.

According to the present process the vapor-phase pulses of the metalsource material and the reducing agent are alternately and sequentiallyfed to the reaction space and contacted with the surface of thesubstrate fitted into the reaction space. The “surface” of the substratecomprises initially the surface of the actual substrate material whichoptionally has been pretreated in advance, e.g., by contacting it with achemical for modifying the surface properties thereof During the growingof the thin films, the previous metal nitride layer forms the surfacefor the following metal nitride layer. The reagents are preferably fedinto the reactor with the aid of an inert carrier gas, such as nitrogen.

Preferably, and to make the process faster, the metal source materialpulse, the reducing boron compound pulse and the nitrogen sourcematerial pulse are separated from each other by an inert gas pulse, alsoreferred to as gas purge in order to purge the reaction space from theunreacted residues of the previous chemical. The inert gas purgetypically comprises an inactive gas, such as nitrogen, or a noble gas,such as argon.

Thus, one pulsing sequence (also referred to as a “cycle”) preferablyconsists essentially of

-   -   feeding a vapor-phase pulse of a metal source chemical with the        help of an inert carrier gas into the reaction space;    -   purging the reaction space with an inert gas;    -   feeding a vapor-phase pulse of a boron source chemical with the        help of an inert carrier gas into the reaction space;    -   purging the reaction space with an inert gas;    -   feeding a vapor-phase pulse of a nitrogen source material into        the reaction space; and    -   purging the reaction space with an inert gas;

The purging time is selected to be long enough to prevent gas phasereactions and to prevent transition metal nitride thin film growth rateshigher than one lattice constant of said nitride per cycle.

The deposition can be carried out at normal pressure, but it ispreferred to operate the method at reduced pressure. The pressure in thereactor is typically 0.01-20 mbar, preferably 0.1-5 mbar. The substratetemperature has to be low enough to keep the bonds between thin filmatoms intact and to prevent thermal decomposition of the gaseousreactants. On the other hand, the substrate temperature has to be highenough to keep the source materials in gaseous phase, i.e., condensationof the gaseous reactants must be avoided. Further, the temperature mustbe sufficiently high to provide the activation energy for the surfacereaction. Depending on the reactants and the pressure the temperature ofthe substrate is typically 200-700° C., preferably 250-500° C.

At these conditions, the amount of reactants bound to the surface willbe determined by the surface. This phenomenon is called“self-saturation”.

Maximum coverage on the substrate surface is obtained when a singlelayer of metal source chemical molecules is adsorbed. The pulsingsequence is repeated until a metal nitride film of predeterminedthickness is grown.

The source temperature is preferably set below the substratetemperature. This is based on the fact that if the partial pressure ofthe source chemical vapor exceeds the condensation limit at thesubstrate temperature, controlled layer-by-layer growth of the film islost.

The amount of time available for the self-saturated reactions is limitedmostly by the economical factors such as required throughput of theproduct from the reactor. Very thin films are made by relatively fewpulsing cycles and in some cases this enables an increase of the sourcechemical pulse times and, thus, utilization of the source chemicals witha lower vapor pressure than normally.

The substrate can be of various types. Examples include silicon, silica,coated silicon, copper metal, and various nitrides, such as metalnitrides. Conventionally, the preceding thin film layer deposited willform the substrate surface for the next thin film.

The present method provides for growing of conformal layers ingeometrically challenging applications. As mentioned above, it ispossible to produce diffusion barriers on dielectric (e.g. silica ornitride) or metal (e.g. copper) surfaces in integrated surfaces. Inthese cases, said surfaces form the substrates for the growing of themetal nitride thin films.

The metal source material can attach on a nitride surface more easily ifthere are certain active groups on the surface. In the following ispresented suggested reaction equations of tungsten hexafluoride (WF₆)for the attaching to silicon wafers.

Silicon wafers have a native oxide on top. The silica (SiO₂) layer maybe just a few molecular layers thick. On a silica surface there are“—OH” groups which can serve as reactive surface sites.WF₆(g)+HO-(ads.)→WF₅—O-(ads.)+HF(g)  (R1)

During the growth process, the metal source compound attaches to thenitride surface. Suggested reaction equations for WF₆ are presented inR2 and R3.WF₆(g)+H—N=(ads.)→WF₅—N=(ads.)+HF(g)  (R2)WF₆(g)+H₂N-(ads.)→WF₅—NH-(ads.)+HF(g)  (R3)

It is of importance that the process parameters are carefully optimisedto protect silicon wafer against corrosion especially during the firstphase of the nitride growth, since the evolved HF gas can attack silicaand form volatile silicon tetrafluoride.SiO₂(s)+4HF(g)→SiF₄(g)+2H₂O(g)  (R4)

The uncovered silicon is prone to further, undesired, reactions.

The metal source materials most typically used are volatile or gaseouscompounds of transition metals, i.e., elements of groups 3, 4, 5, 6, 7,8, 9, 10, 11 and/or 12 (according to the system recommended by IUPAC) inthe periodic table of elements. In particular, the film consistsessentially of W, Ti, Zr, Hf, V, Nb, Ta, Cr and/or Mo nitride(s) andthus gaseous or volatile compounds of these are preferably used in themethod of the present invention.

Since the properties of each metal compound vary, the suitability ofeach metal compound for the use in the process of the present inventionhas to be considered. The properties of the compounds are found, e.g.,in N. N. Greenwood and A. Earnshaw, Chemistry of the Elements, 1^(st)edition, Pergamon Press, 1986.

The metal source material (as well as the reducing boron compound andthe nitrogen source material) has to be chosen so that the requirementsfor sufficient vapor pressure, the above-discussed criteria ofsufficient thermal stability at substrate temperature and sufficientreactivity of the compounds are fulfilled.

Sufficient vapor pressure means that there must be enough sourcechemical molecules in the gas phase near the substrate surface to enablefast enough self-saturated reactions at the surface.

In practice, sufficient thermal stability means that the source chemicalitself must not form growth-disturbing condensable phases on thesubstrates or leave harmful levels of impurities on the substratesurface through thermal decomposition. Thus, one aim is to avoidnon-controlled condensation of molecules on substrates.

Further selecting criteria include the availability of the chemical athigh purity, and the ease of handling, inter al., without severeprecautions.

Typically, suitable metal source materials can be found among halides,preferably fluorides, chlorides, bromides or iodides, or metal organiccompounds, preferably alkylaminos, cyclopentadienyls, dithiocarbamatesor betadiketonates of desired metal(s).

According to a preferred embodiment of the invention, tungsten nitride(W_(x)N_(y), referred to as WN hereinafter) is grown. Then, tungstensource chemical is a tungsten compound selected according to the abovecriteria. Preferably, the tungsten source material is selected from thegroup comprising

-   -   a halide such as WF_(x), WCl_(y), WBr_(m) or WI_(n) wherein x,        y, m and n are integers from 1 to 6, in particular WF₆;    -   a carbonyl such as tungsten hexacarbonyl W(CO)₆ or        tricarbonyl(mesitylene)tungsten;    -   cyclopentadienyl such as bis(cyclopentadienyl)tungsten        dihydride, bis(cyclopentadienyl)tungsten dichloride or        bis(cyclopentadienyl)ditungsten hexacarbonyl; and    -   β-diketonate.

According to a preferred embodiment transition metal nitrides are mixedso that in the growing process two or more different metal sourcematerials are used. For example, tungsten nitride can be mixed with TiN.

The metal reactant will react with the substrate surface forming a(covalent) bond to the surface bonding groups. The adsorbed metalspecies will contain a residue of the reactant compound, such as halogenor hydrocarbon. According to the present invention, this residue reactswith the gaseous boron compound, which reduces the metal compound on thesurface.

The reducing strengths of the boron compounds vary. Thus, some boroncompounds may reduce the metal compound to elemental metal, and othersto a certain oxidation state. It is important that only those metalswhich are reactive with the nitrogen compounds also in their elementalform are reduced to metals. Typically, the oxidation state of the metalsource compound is reduced so that the metal on the surface is in a formof a compound. The metal compounds react with the nitrogen sourcematerials easily forming metal nitrides.

The boron sources are selected bearing in mind the same criteria as forthe metal source materials. In general, the boron compound can be anyvolatile, thermally sufficiently stable and reactive boron compoundcapable of reducing the metal species bonded to the surface.

The reactions of different metal source materials with one and samereducing agent (and vice versa) can lead to different reaction(by)products. According to the present invention, the metal sourcematerial and boron compound are selected so that the resulting boroncompound(s) is (are) gaseous. By this is meant that the compound formedis gaseous enough to be moved from the reaction space with the aid ofthe inert purging gas, and, on the other hand, does not decompose, e.g.,catalytically or thermally, to condensable species. In all, byproductswill not remain as impurities in the films. If a reactive site on thesurface is contaminated, the growing rate of the film decreases. Byselecting the metal source material(s) and boron compound as indicatedabove, the growing rate of the film does not essentially decrease, i.e.,decreases by a maximum of 0.1%, preferably by less than 0.01%, and inparticular by less than 0.001% in each cycle. An example of anunsuitable pair is TiCl₄ and triethyl boron, the reaction thereof notleading to desired results.

The selection can be facilitated with computer programs having asufficiently extensive thermodynamics database, which enables to checkthe reaction equilibrium and thus predict which reactants havethermodynamically favorable reactions. An example of this kind ofprograms is HSC Chemistry, version 3.02 (1997) by Outokumpu Research Oy,Pori, Finland.

A vast range of boron chemicals makes it possible to choose suitablereducing strength and avoid boride formation. It is possible to use oneor more boron compounds in the growing of one and same thin film.

Preferably, one or more of the following boron compounds is used:

Boranes having formula (I)B_(n)H_(n+x),wherein n is an integer from 1 to 10, preferably from 2 to 6, and

-   -   x is an even integer, preferably 4, 6 or 8,        or formula (II)        B_(n)H_(m),  (II)        wherein n is an integer from 1 to 10, preferably from 2 to 6,        and    -   m is an integer different than n, m being from 1 to 10,        preferably from 2 to 6.

Boranes according to formula (I) are exemplified by nido-boranes(B_(n)H_(n+4)), arachno-boranes (B_(n)H_(n+6)) and hypho-boranes(B_(n)H_(n+8)). Of the boranes according to formula (II), examplesinclude conjuncto-boranes (B_(n)H_(m)). Also borane complexes, such as(CH₃CH₂)₃N.BH₃ can be used.

Borane halides, particularly fluorides, bromides and chlorides. As anexample of a suitable compound B₂H₅Br should be mentioned. It is alsopossible to use borane halide complexes

Boron halides with high boron/halide ratio, such as B₂F₄, B₂Cl₄ andB₂Br₄.

Halogenoboranes according to formula (III)B_(n)X_(n),  (III)wherein X is Cl or Br and

-   -   n=4, 8-12 when X=Cl, and    -   n=7-10 when X=Br

Carboranes according to formula (IV)C₂B_(n)H_(n+x),  (IV)wherein n is an integer from 1 to 10, preferably from 2 to 6, and

-   -   x is an even integer, preferably 2, 4 or 6.

Examples of carboranes according to formula (IV) includecloso-carboranes (C₂B_(n)H_(n+2)), nido-carboranes (C₂B_(n)H_(n+4)), andarachno-carboranes (C₂B_(n)H_(n+6)).

Amine-borane adducts according to formula (V)R₃NBX₃,  (V)wherein R is linear or branched C₁-C₁₀, preferably C₁-C₄ alkyl or H, and

-   -   X is linear or branched C₁-C₁₀, preferably C₁-C₄ alkyl, H or        halogen,

Aminoboranes where one or more of the substituents on B is an aminogroup according formula (VI)R₂N,  (VI)wherein R is linear or branched C₁-C₁₀, preferably C₁C₄ alkyl orsubstituted or unsubstituted aryl group.

An example of suitable aminoborane is (CH₃)₂NB(CH₃)₂.

Cyclic borazine (—BH—NH—)₃ and its volatile derivatives.

Alkyl borons or alkyl boranes, wherein the alkyl is typically linear orbranched C₁-C₁₀ alkyl, preferably C₂-C₄ alkyls. Particularly preferredis triethyl boron (CH₃CH₂)₃B, because it is easily vaporized.

Particularly preferred boron compound is triethyl boron (CH₃CH₂)₃B.

The reduced metal species bound on the substrate surface will then besubjected to reaction with a nitrogen-containing compound. The nitrogencompound used as the nitrogen source material is volatile or gaseous andchosen according to the above criteria, including the criterion relatingto the reaction byproducts.

Preferably, the nitrogen compound is selected from the group comprising

-   -   ammonia (NH₃) and its salts, preferably halide salt, in        particular ammonium fluoride or ammonium chloride;    -   hydrogen azide (HN₃) and the alkyl derivates of the said        compound such as CH₃N₃;    -   hydrazine (N₂H₄) and salts of hydrazine such as hydrazine        hydrochloride;    -   alkyl derivates of hydrazine such as dimethyl hydrazine;    -   nitrogen fluoride NF₃;    -   hydroxyl amine (NH₂OH) and its salts such as hydroxylamine        hydrochloride;    -   primary, secondary and tertiary amines such as methylamine,        diethylamine and triethylamine; and    -   nitrogen radicals such as NH₂*, NH** and N***, wherein * means a        free electron capable of bonding, and excited state of nitrogen        (N₂*).

When no reducing agent is used, the nitride film resulting from theabove-described process has a N/W molar ratio of greater than 1, i.e.,the nitride is mostly in the form WN₂. When operating without a reducingagent, it is also possible to feed the nitrogen source material pulseinto the reaction space first and the transition metal source materialsecond, i.e., use a reverse order of source material pulses. Thedeposition process ends also in this case with a nitrogen sourcematerial pulse. Thus, the structure of the film is different from theone obtained by a process otherwise similar but employing a reducingagent. The film produced according to the process employing no reducingagent, has rather high resistivity.

The following non-limiting examples illustrate the invention.

EXAMPLE 1

Tungsten hexafluoride (WF₆) and ammonia (NH₃) were used as sourcechemicals. Both chemicals are liquefied gases at room temperature andposses high enough vapor pressure without additional heating for the ALDprocess. Source tubing and the reactor were purged with nitrogen gaswhich had a purity of 99.9999% (i.e. 6.0). The N₂ gas was prepared fromliquid nitrogen. A 200-mm silicon wafer was loaded to an ALD reactor asdescribed in Finnish Patent No. 100409 of assignee. Source chemicalswere pulsed alternately to the substrates at the reaction chamber. Thedeposition was started and ended with an NH₃ pulse. The pulsing cycleconsisted of the following steps:

-   NH₃ vapor pulse 0.5 s-   N₂ gas purge 1.0 s-   WF₆ vapor pulse 0.25 s-   N₂ purge 0.8 s

The pulsing cycle was repeated for 500 times which produced a 30 nm filmwith the typical growth rate of 0.6 Å/cycle. The composition, impuritiesand the thickness of the resulting thin film were analyzed by ElectronDiffraction Spectroscopy (referred to as RDS hereinafter). EDS showed anN/W ratio of 1.3 which means that the phase of tungsten nitride wasbetween WN and WN₂, i.e. rich in nitrogen. Decreasing the growthtemperature from 400° C. to 360° C. increased the fluorine content from2 wt.-% to 4 wt.-%.

The resistivity of the tungsten nitride film was obtained by combiningthe thickness value with the four-point probe measurements. Theresistivity of the film grown at 400° C. was 1900 μΩcm. High resistivitywas possibly caused by the high nitrogen content of the film.

EXAMPLE 2

Tungsten hexafluoride (WF₆), triethylboron (CH₃CH₂)₃B and ammonia (NH₃)were used as source chemicals. All the chemicals are liquids orliquefied gases at room temperature and poses high enough source vaporpressure without additional heating for the ALD process. Source tubingand the reactor were purged with nitrogen gas which had a purity of99.9999% (i.e. 6.0). The N₂ gas was prepared from liquid nitrogen. A200-mm silicon wafer was loaded to an F200 ALD reactor. Source chemicalswere pulses alternately to the substrates at the reaction chamber. Thepulsing cycle consisted of the following steps:

-   WF₆ vapor pulse 0.25 s-   N₂ purge 0.8 s-   (CH₃CH₂)₃B vapor pulse 0.01 s-   N₂ gas purge 0.5 s-   NH₃ vapor pulse 0.25 s-   N₂ gas purge 0.5 s

The pulsing cycle was repeated for 500 times resulting in a 30-nm filmat 360° C. The samples were analyzed by EDS for thickness andcomposition. The thin film consisted of tungsten and nitrogen whileboron could not be seen in detectable amounts. There was 3 wt.-% offluorine as an impurity in the film. The resistivity of the tungstennitride film was obtained by combining the thickness value with thefour-point probe measurements. The resistivities were 130-160 μΩcm.

The inventors assume that the boron chemical acted as a reducing agentand removed fluorine from tungsten fluoride. The benefit of this boronchemical is that possible byproducts such as BF₃ and CH₃CH₂F are gaseousat the deposition temperature and do not disturb the nitride growth.

1. An atomic layer deposition (ALD) process for growing a metal nitride thin film on a substrate comprising alternately and sequentially contacting a substrate in a reaction space with vapor phase pulses of: a metal source material that forms a monolayer on the substrate surface; a boron compound that reduces the metal source material on the substrate surface; and a nitrogen source material that reacts with the reduced metal source material, wherein an inert gas is provided to the reaction space after every pulse.
 2. An atomic layer deposition (ALD) process for growing a metal nitride film on a substrate in a reaction space comprising the sequential steps of: a) feeding a vapor-phase pulse of a metal source chemical into the reaction space with an inert carrier gas; b) purging the reaction space with an inert gas; c) feeding a vapor-phase pulse of a boron source chemical into the reaction space with an inert carrier gas; d) purging the reaction space with an inert gas; e) feeding a vapor-phase pulse of a nitrogen source chemical into the reaction space; f) purging the reaction space with an inert gas; and g) repeating steps a) through f) until a metal nitride film of a desired thickness is formed.
 3. The process of claim 2, wherein the metal in the metal source chemical is selected from the group consisting of W, Mo, Cr, Ta, Nb, V, Hf, Zr and Ti.
 4. The process of claim 2, wherein the metal source chemical is selected from the group consisting of metal halides and metal organic compounds.
 5. The process of claim 4, wherein the metal source chemical is selected from the group consisting of metal fluorides, metal chlorides, metal bromides and metal iodides.
 6. The process of claim 4, wherein the metal source chemical is selected from the group consisting of alkylamino compounds, cyclopentadienyl compounds, dithiocarbamate compounds and betadiketonate compounds.
 7. The process of claim 2, wherein the metal source chemical is a tungsten compound selected from the group consisting of tungsten halides, tungsten carbonyls, tungsten cyclopentadienyls, and tungsten β-diketonates.
 8. The process of claim 7, wherein the metal source chemical is selected from the group consisting of WF_(x), WCl_(y), WBr_(m) and WI_(n) wherein x, y, m and n are integers from 1 to
 6. 9. The process of claim 8, wherein the metal source chemical is WF₆.
 10. The process of claim 7, wherein the metal source chemical is selected from the group consisting of tungsten hexacarbonyl (W(CO)₆) and tricarbonyl(mesitylene)tungsten.
 11. The process of claim 7, wherein the metal source chemical is selected from the group consisting of bis(cyclopentadienyl)tungsten dihydride, bis(cyclopentadienyl)tungsten dichloride and bis(cyclopentadienyl)ditungsten hexacarbonyl.
 12. The process of claim 2, wherein the boron source chemical is selected from the group consisting of boranes of formula (I) B_(n)H_(n)+_(x),  (I) wherein is an integer from 1 to 10 and x is an even integer; boranes of formula (II) B_(n)H_(m),  (II) wherein n is an integer from 1 to 10 and m is an integer different than n, m being from 1 to 10; and complexes thereof.
 13. The process of claim 12, wherein the boron source chemical is selected from the group consisting of boranes of formula (I) B_(n)H_(n)+_(x),  (I) wherein n is an integer from 2 to 6, and x is 4, 6 or 8; boranes of formula (II) B_(n)H_(m),  (II) wherein n is an integer from from 2 to 6, and m is an integer different than n, m being from 2 to 6; and complexes thereof.
 14. The process of claim 12, wherein the boranes are selected from the group consisting of nido-boranes of formula B_(n)H_(n+4), arachno-boranes of formula B_(n)H_(n+6), hypho-boranes of formula B_(n)H_(n+8), and conjuncto-boranes B_(n)H_(m), wherein n is an integer from 1 to 10 and m is an integer from 1 to 10 that is different than n.
 15. The process of claim 2, wherein the boron source chemical is selected from the group consisting of carboranes according to formula (IV) C₂B_(n)H_(n+x),  (IV) wherein n is an integer from 1 to 10 and x is an even integer.
 16. The process of claim 15, wherein n is an integer from 2 to 6, and x is 2, 4 or
 6. 17. The process of claim 15, wherein the carboranes are selected from the group consisting of closo-carboranes (C₂B_(n)H_(n+2)), nido-carboranes (C₂B_(n)H_(n+4)) and arachno-carboranes (C₂B_(n)H_(n+6)), wherein n is an integer from 1 to
 10. 18. The process of claim 2, wherein the boron source chemical is selected from the group consisting of amine-borane adducts according to formula (V) R₃NBX₃,  (V) wherein R is linear or branched C₁-C₁₀, and X is linear or branched C₁-C₁₀.
 19. The process of claim 18, wherein R is selected from the group consisting of linear or branched C₁-C₄ alkyl and H, and X is selected from the group consisting of linear or branched C₁-C₄ alkyl, H and a halogen.
 20. The process of claim 2, wherein the boron source chemical is selected from the group consisting of aminoboranes wherein one or more of the substituents on B is an amino group according to formula (VI) R₂N,  (VI) wherein R is linear or branched C₁-C₁₀.
 21. The process of claim 20, wherein R is selected from the group consisting of a C₁-C₄ alkyl group, a substituted aryl group and an unsubstituted aryl group.
 22. The process of claim 2, wherein the boron source chemical is selected from the group consisting of alkyl borons and alkyl boranes, wherein the alkyl is a linear or branched C₁-C₁₀ alkyl.
 23. The process of claim 22, wherein the alkyl is a linear or branched C₂-C₄ alkyl.
 24. The process of claim 2, wherein the boron source chemical is a boron halide.
 25. The process of claim 24, wherein the boron source chemical is selected from the group consisting of B₂F₄, B₂Cl₄ and B₂Br₄.
 26. The process of claim 2, wherein the boron source chemical is selected from the group consisting of halogenoboranes of formula (III) B_(n)X_(n),  (III) wherein X is Cl or Br, n=4, 8-12 when X=Cl and n=7-10 when X=Br.
 27. The process of claim 2, wherein the boron source chemical is selected from the group consisting of cyclic borazine (—BH—NH—)₃ and the volatile derivatives thereof.
 28. The process of claim 2, wherein the boron source chemical is selected from the group consisting of borane halides and complexes thereof.
 29. The process of claim 2, wherein the nitrogen source chemical is selected from the group consisting of ammonia (NH₃) and its salts, hydrogen azide (HN₃) and the alkyl derivates thereof, hydrazine (N₂H₄) and salts of hydrazine, alkyl derivates of hydrazine, nitrogen fluoride NF₃, hydroxyl amine (NH₂OH) and salts thereof, primary, secondary and tertiary amines, nitrogen radicals, and excited state nitrogen (N₂*), wherein * is a free electron capable of bonding.
 30. The process of claim 29, wherein the nitrogen source chemical is selected from the group consisting of ammonium fluoride, ammonium chloride, CH₃N₃, hydrazine hydrochloride dimethyl hydrazine, hydroxylamine hydrochloride, methylamine, diethylamine and triethylamine.
 31. The process of claim 2, wherein the substrate comprises one or more materials selected from the group consisting of silicon, silica, coated silicon, copper metal and nitride.
 32. The process of claim 2, wherein the metal nitride thin film comprises a diffusion barrier in an integrated circuit. 